1. Field of the Invention
The invention relates to an antenna configuration for motor vehicles, the roof region of which is made predominantly of dielectric materials, and for motor vehicles having roof structures that can be collapsed for open driving conditions.
2. The Prior Art
An antenna is essentially defined by the emitter as a wave-type converter (free-space wave to line-conducted wave or vice versa), which is electrically effective for a certain frequency range because of its geometrical configuration, and is in resonance; the feed point as the interface between emitter and line, at which the wave is coupled in and out, respectively; as well as the ground reference or counter-pole.
The function of an antenna is represented by means of the radiation characteristics (viewed spatially), or by means of radiation diagrams (in vertical or horizontal section).
For motor vehicles having metal bodies with an essentially unchangeable body structure, the placement of antennas and their function in interaction with the aforementioned components present no problems. The antennas in a rear window surrounded by sheet-metal surfaces, the rod antennas on the metal roof of a sedan, and others, can be positioned and optimized for stable function without any great difficulties, with regard to the implementation of the emitters as well as the assignment of the feed point and a sufficient ground connection, taking into consideration the high frequency (HF)-related specifics of the motor vehicle body, in each instance.
It is different for vehicles having variable body components and for vehicles having body parts made of dielectric materials.
In the present context, the variability relates to the roof construction, specifically in the case of convertibles. Nowadays, a differentiation is made, in the case of convertibles, between a folding roof made of metal elements, and the conventional structure made of a metal frame of connecting rods covered with fabric or plastic surfaces.
For such vehicles as well, rear windows having integrated antenna structures are known. These antennas can be used for radio services as long as the roof does not disappear under the cover or in the trunk in the collapsed state.
For the case of the open roof, rod antennas, in most cases on the fenders, or antennas in the bumper area are generally used as an alternative.
The rod antennas are exposed to vandalism and theft. The antennas in the bumper, on the other hand, are highly direction-oriented, with the sheet-metal body “at their back,” and one should place at least two emitters in opposite directions on the vehicle, combined with diversity or phase-regulated signal integration. In this connection, long HF lines between the individual feed points and to the receiver are then unavoidable. Fundamentally, placement at a low height in the bumper is also not the most advantageous for reception.
For a rear window that is surrounded by a fabric cover, on the other hand, a ground connection can be a problem. Possible solutions would be a ground-plane arrangement or dipole arrangement. Something like those solutions would be complicated, however, and would limit the window area that is available for a multi-antenna arrangement, for example.